As described in WO 97/22597 A1, WO 98/57954 A1 and WO 00/35915 A1, there have been known several piperazine derivatives having pharmacological activities such as an antagonism against tachykinin and the like.
A problem of the invention is to provide a novel process for preparing a useful piperazine derivative, namely Compound I, which has pharmacological activities such as an antagonism against tachykinin, particularly an antagonism against substance P, an antagonism against neurokinin A, an antagonism against neurokinin B and the like.